Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
_ V095/25703 ~1 8 ~ I 3`1 PCT~S95/03187
SILICON NITRIDE BEARING B~TT HAVING HIGH FATIGUE LIFE
Advanced structural ceramic materials have gained the
attention of industry by virtue of their superior
performance qualities. These qualities, such as superior
high temperature strength, high tollghn~Cc~ and resistance
to thermal shock and oxidation, provide the bases for their
potential use in a variety of applications.
U.S. Patent No. 4,935,388 (Lucek) suggests that the
reliability of a ceramic material can be correlated with
its spatial homogeneity, and that such homogeneity can be
characterized by the extent to which the material exhibits
optical anomalies. In particular, Lucek teaches that the
reliability of silicon nitride greatly improves when the
material is spatially homogeneous, e.g., has no optical
anomalies greater than about 70 microns. Lucek suggests
that the above optical anomalies signify a number of
inhomogeneous phenomena, including, but not limited to,
regions of porosity, regions of contamination from
materials such as iron, and regions having microcracks.
The silicon nitride ceramics disclosed by Lucek, which
contained about l w/o MgO as a sintering aid, have a
rolling contact fatigue life ("RCF life") defined by an Ll0
value of at least four million stress cycles in ASTM test
STP 771 under 6.9 GPa applied contact stress. Although
this RCF life is one of the highest known, commercial
desire for silicon nitride materials having improved RCF
life continues.
Accordingly, it is the object of the present invention
to provide a silicon nitride material having a superior RCF
life.
SUMMARY OF THE PRESENT lNv~NllON:
In accordance with the present invention, there is
provided a bearing ball consisting essentially of:
a) at least about 94 w/o of a crystalline phase of
silicon nitride, and
b) a single grain boundary phase consisting
essentially of Mg, Al, Si and O.
W095/25703 ~18 ~ 13 1 J' PCT~S95/03187
In preferred embodiments, the grain boundary phase
consists essentially of between l and 2 w/o Mg, as
magnesia; between 0.2 and l.0 w/o Al, as alumina; between
2 and 4 w/o Si, as silica; and oxygen. More preferably, the
Mg component of the grain boundary phase is between l.0 and
l.6 w/o, as magnesia, the Al component of the grain
boundary phase is between 0.3 and 0.6 w/o, as alumina, and
the Si component of the grain boundary phase is between 2.0
and 3.0 w/o, as silica, of the bearing. In especially
preferred emho~;ments, the bearing has an Ll0 value of
between 50 and 80 million stress cycles in ASTM test 771
under 6.9 GPa applied contact stress.
DET~TT~n DESCRIPTION OF THE PRESENT lNV~NllON:
It has been unexpectedly found that adding between
about 0.20 w/o and about l.0 w/o alumina to a green body
consisting essentially of silicon nitride and about l w/o
MgO results in a ceramic having an l~neYr~ctedly high RCF
life.
Without wishing to be tied to a theory, it is believed
that, during the sintering of the conventional silicon
nitride ceramics of Lucek, the MgO sintering aid and the 3
w/o silica (present in silicon nitride power as an
impurity) formed at least two immiscible liquid phases at
equilibrium, resulting in at least two grain boundary
phases in the sintered body. This non-uniform grain
boundary phase probably degraded the strength, toughness
and RCF life of the ceramic. The alumina addition of the
present invention is believed to provide a single NgO-SiO2-
Al2O3 phase at equilibrium, resulting in a single grain
boundary phase according to a phase diagram. Hence, more
uniform and higher properties result.
The silicon nitride ceramic of the present invention
can be made from typical materials and conventional
processing steps. In preferred embodiments, the ceramic is
made from a silicon nitride powder or a precursor thereof.
If the ceramic is made from a silicon nitride powder, any
typical silicon nitride powder can be used. Typically,
silicon nitride comprises at least about 94 w/o of the
~095/25703 2 1 8 S 1 3 1 ; PCT~S95/03187
ceramic of the present invention, preferably between about
97 and about 98.5 w/o.
In preferred embodiments of the present invention,
magnesia and alumina are used as sintering aids. Magnesia
is added in amounts of from about 1.0 w/o to about 2.0 w/o,
preferably between about 1.0 and about l.6 w/o of the
sintered ceramic. Most preferably, about 1 w/o of MgO,
having a purity of more than 99% and an average particle
size of less than 1 micron, is added. Similarly, alumina
may be added in amounts of from about 0.20 w/o to about 1.0
w/o, more preferably between about 0.4 and about 0.6 w/o of
the sintered ceramic. Most preferably, about 0.47 w/o of
Al203, having a purity of more than 99~ and an average
particle size of less than 1 micron, is added.
Silica may be present in the starting silicon nitride
powder as an impurity. As a result, silicon may be present
in the grain boundary phase of the sintered ceramic in an
amount of between about 2 and 4 w/o, preferably 2 and 3 w/o
of the ceramic, as silica.
The silicon nitride and sintering aid powders may be
mixed by any known mixing process including, but not
limited to, ball milling and attrition milling. In
preferred emho~;ments of the present invention, vibratory
milling is preferred.
If a green body is formed, green body formation may be
performed by any typical process used in art. These
processes include slip casting, injection molding, freeze
casting and cold isostatic pressing. In preferred
embodiments of the present invention, the powders of the
present invention are cold isostatic pressed.
The sintering cycle of the present invention may
include any conventional sintering ~lo~e-~s, including
pressureless sintering, gas pressure sintering, hot
pressing and hot isostatic pressing using glass ("glass
encapsulated hipping"). In preferred, embodiments of the
present invention, the glass encapsulation hipping
processes as described in U.S. Patent Nos. 4,446,100, and
4,339,271 are used. It is further preferred that
W095/25703 2 1 8 5 1 3 ~ ; PCT~S95/03187 _
embodiments utilizing U.S. Patent Nos. 4,446,110 and
4,339,271 be practiced at very high packing efficiency,
i.e., with parts touching each other.
EXAMPLE I
Silicon nitride powder with the following
characteristics was used as the powder in this example:
more than about 90 w/o alpha Si3N4, less than about 2.0 w/o
total oxygen, less than 0.1 w/o calcium, about 0.03 w/o
iron, about 0.05 w/o aluminum, and a surface area of about
5 - 9 m2/g
About 98 parts by weight of this powder was combined
with about 1.33 parts by weight of reagent grade magnesium
carbonate and about 0.47 parts by weight alumina and
isopropyl alcohol to form a slurry having 45% solids. The
slurry was simultaneously blended and milled in a vibratory
mill with Si3N4 milling media. The final surface area was
about 10 - 14 m2/g. The resultant slurry was p~sc~ through
a 20 micron screen and a magnetic separator. The slurry
was concentrated to 65% solids by cross-flow filtration.
Next, PVP (polyvinylpyrrolidone) was added to the slurry in
an amount of about 1.25 w/o of the powder. The slurry was
then dried in an explosion-proof spray dryer. The dried
powder was then passed through a 30 mesh nalgene screen.
All of the above operations were performed in a class 1000
clean room dedicated to silicon nitride processing. The
dried agglomerated powder was cold isostatically pressed at
30 ksi into cylinders and ground into rods having a
diameter of about 0.52 inches and a length of about 3.8
inches. The powder compacts were air fired at 600C to
remove the PVP. Samples were encapsulated in glass media
and hot isostatically pressed at 30,000 psi (20.7 MPa)
pressure for about 1 hour at about 1790C.
EXAMPLES II and III
The above process was repeated essentially
identically, except that % inch balls and 3 x 4 x 50 mm
flexure rods were produced.
The resulting ceramics were measured for hardness.
Vickers diamond pyramid hardness indentations from 10 kg
_ Voss/25703 21~S131 : : PCT~S95/03187
loads were created. The mean hardness was found to be
about 15.2 GPa. In comparison, NBD-200, a 1 w/o MgO
silicon nitride bearing material manufactured by the Norton
Company of Worcester, MA, has a hardness of about 15 - 15.6
GPa.
Fracture toughn~-cc was determined by the fracture of
4 point bend specimens of 3 x 4 x 50 mm contAin;ng a
Vickers indentation (10 kg load) on the tensile surfaces
within the inner span, at a crosshead speed of about 0.5
mm/min, according to procedures described by P. Chantikul
et al., in "A Critical Evaluation of Indentation
Techniques for Measuring Fracture Tollghnec~ Strength
Methods", J. Am. Ceram. Soc. 64(9), pp. 539-544 (1981).
The average toughness of the ceramics of the present
invention was about 5.6 MPa m~. In comparison, NBD-200 has
a fracture toughness of about 5 - 5.8 MPa m~.
The 4-point flexure strength of the ceramics of the
present invention were also determined. 3 x 4 x 50 mm type
B specimens on a 40 mm outer span and a 20 mm inner span
test jig (ASTM C1161-90) were selected. The mean flexure
strength was determined to be about 950 MPa. In
comparison, NBD-200 has a flexural strength of about 700 -
850 MPa.
Tensile strength was also analyzed by the ASCERA
Tensile Test Procedure. This procedure is set out in Brit.
Ceram. Trans J., 89, 21-23, 1990. The tensile strength of
the present invention was found to be about 475 MPa. In
comparison, NBD-200 has a tensile strength of about 400
MPa.
The rods made in Example II were subjected to rolling
contact fatigue (RCF) tests, specifically the accelerated
bearing test procedure described in ASTM-STP 771. This
test procedure applies contact pressure higher than that
normally encountered in service in order to accelerate the
fatigue failure of the test material. The test uses
statistical analysis of data from numerous test specimens,
and the results are normally reported in terms of a
variable Lx, which denotes the number of stress cycles
Wo95/25703 218 51 31 PCT~S95/03187
which cause failure in X percent of the specimens tested at
a given stress level. In carrying out the RCF test for the
present invention, three slave balls made of AISI 52100
steel were loaded against a rod consisting of the silicon
nitride of the present invention at an average contact
stress of about 6.9 GPa. The rod was rotated at about 3600
rpm with an electric motor. Both the slave balls and the
rod were lubricated with lubricating oil supplied by a drip
feeder at a rate of about 8 drops per minute. The test
produced circumferential fatigue tracks in the rod which
were about 0.1 ;n~h~c in width. The silicon nitride of the
present invention displayed normal failure modes and
uniform wear. Of the 24 rods of the present invention
which were subjected to RCF testing, the first rod to fail
did so at about 54 million cycles. If a Weibull slope of
about 0.74 is assumed (i.e., it is assumed the present
invention has the same failure meçhAni~m as NBD-200), an
L10 of about 80 million cycles is realized for the present
invention. In comparison, NBD-200 has an L10 RCF life of
4 million cycles.
The meçhAnical characteristics of the present
invention as exemplified by Examples I - III are summarized
in Table 1, along with comparative silicon nitride ceramic
NBD-200.
TABLE 1
P~r~,~ E8ANPLES I - III NBD-200
L10 RCF Life (cycles) 80,000,0004,000,000
Tensile Strength (MPa) 475 400
Flexural strength (MPa) 950 <850
Fracture Toughn~cc 5.6 < 5.8
(MPa m~)
Hardness (GPa) 15.2 <15.6
The silicon nitride of the present invention may be
21~131
VO 95/25703 - ~ ~ ` PCT~US95/03187
used in a number of conventional ceramic applications,
including but not limited to, ball bearings, roller
bearings, plain sliding bearings and other structural or
wear applications.
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